The present invention relates to a transparent polyurethane foam wall containing microorganisms, and a process for the preparation thereof and its use in a biophotoreactor.
Over the past few years, new procedures have been developed for culturing microorganisms able to grow under the action of light, in the presence of carbon dioxide gas and an appropriate culture medium, for forming products having an economic interest in connection with foods, pharmacy and the assisted recovery of petroleum.
Thus, certain microorganisms, e.g. the microrhodophyceae Porphyridium cruentum, is able to excrete sulphated polysaccharides having a significant economic value. By culturing microorganisms such as Botryococcus braunii, it is possible to obtain hydrocarbons which are also of significant interest.
In order to obtain such microorganism cultures, it is necessary to bring them into contact with an appropriate nutrient liquid medium and with carbon dioxide gas and to simultaneously expose the microorganisms to sunlight or artificial light. The product synthesized by the microorganism can then be extracted from the nutrient liquid medium. These cultures are generally produced in biophotoreactors designed so as to ensure an irradiation of the microorganisms by sunlight or artificial light, and a contacting thereof with an appropriate medium.
For example, a biophotoreactor of this type is described in U.S. Pat. No. 3,955,317 published on May 11th 1976. According to this patent, the biophotoreactor is constituted by a tubular system which is transparent to solar radiation, which contains microalgae suspended in a nutrient liquid into which is injected carbon dioxide gas. The vegetable cells bring about the photopolymerization of the carbon dioxide gas into a desired chemical product, e.g. a product having a food or pharmaceutical use, or into a hydrocarbon, as a function of the microalgae used, and said reaction is also accompanied by a release of oxygen. To prevent premature sedimentation of the celluar biomass within the reactor, it is necessary to keep the latter in motion. This can be brought about by using a tubular system which is open at both ends and by continuously injecting into the intake a mixture of nutrient liquid and microalgae, the mixture being extracted from the outlet. The microalgae and the metabolites produced by the latter are then separated from the liquid medium by centrifuging.
The use of a biophotoreactor of this type suffers from certain disadvantages because it requires a large amount of energy on the one hand for circulating the microalgae, and on the other for separating the metabolites, which increases the cost of the thus produced products.
In order to reduce the energy used, consideration has been given to the immobilization of the microalgae in a spongy polyurethane foam structure, such as that described in the report of the Academie des Sciences of 6.7.1981, vol. 293, series III, pp. 35-37 and entitled "Production of sulphated polysaccharides by a biophotoreactor having immobilized Porphyridium cruentum cells". In this case, the microorganisms are introduced at the time of preparing the polyurethane foam by mixing in substantially equal parts the polyurethane foam precursor compositions and a suspension of algae in a nutrient liquid. The formation of the foam and the immobilization of the microalgae within the latter take place simultaneously within a few dozen minutes at ambient temperature. The foam is then cut into small dices of approximate side length 1.5 cm. They are then introduced into a glass column containing a liquid nutrient medium and are exposed to sunlight.
The microalgae which survive the polymerization then recolonize all the pores of the polyurethane dices, which are continuously irrigated by the liquid nutrient medium and by the air containing 2% carbon dioxide gas. As in the previous case, the microalgae absorb solar radiation and the carbon from the carbon dioxide gas and produce hydrosoluble polysaccharides which they reject through the pores into the liquid nutrient medium. Although this type of reactor makes it possible to considerably reduce energy costs, it still suffers from certain disadvantages.
Thus, the polyurethane dices are not very transparent and therefore the solar radiation only weakly reaches those dices located in the centre of the tube. Moreover, difficulties are encountered in transferring the materials (carbon dioxide gas supply and polysaccharide extraction).